Failures that are caused by the fatigue loads on the structural parts could be followed by fatal accidents. Mechanical parts can fail unexpectedly due to fatigue loads and result in loss in life and property. Several fatigue design techniques have been developed in order to bring a solution to this problem. The best-known method is the ASME holistic design methodology. In this methodology, during the design phase, working conditions are taken into consideration so that fatigued design is operational for a specific lifetime. However, this model does not guarantee that mechanical parts would work under the assumed conditions or stay undamaged during the assumed lifetime of the structure. Hence, fatigue failure has been a problem and leads to fatal accidents in a variety of mechanical parts.
Observing real-time fatigue damage is important in order to set up a correct balance between performance and structural integrity of mechanical systems. Several methods have been developed to detect fatigue damage on critical mechanical parts and structural elements working under cyclic loads. In model analysis technique, obtaining information about fatigue damage is possible by observing the variations of frequency and vibration models, originated by cracks in the mechanical parts. Another technique to determine the damages resulted by the stress concentrations is wave propagation. In this method, the response of the original system is observed in the frequency domain and the values are compared to the fatigued version of the system to reveal fatigue damage. Yet another method is based on measuring stress values synchronously on critical points of the structure (such as in the joints of aircraft wings) and therefore projecting fatigue effects that are resulted by these.
It is utmost critical to monitor structural integrity of safety-critical structures like bridges, aircraft, naval ship etc. as they age in service. Failure to do so results in catastrophic failure of the structure during its service life, which may cause damage to lives and property. Performing manual inspection of structures by human inspectors is not only difficult but also proved to be unsatisfactory as it is easy to miss problem spots. Because of its importance, there have been many research and countless inventions toward this subject in the literature.
Boller and Meyendorf reviewed the state of the art in structural health monitoring in an article titled “State of the art in structural health monitoring for aeronautics” in 2008 in Proceedings of International Symposium on NDT (non destructive testing) in Aerospace. The article lists state of the art of sensors used for aircraft health monitoring applications and lists sensors like; electrical strain gauges, electrical crack wires, acoustic emission, acousto-ultrasonics, laser vibrometry, comparative vacuum monitoring (CVM), optical fiber Bragg grating (FBG), MEMS and electromagnetic foils. These sensors are designed toward sensing existence of the cracks that develop during lifetime of the structure.
Another review report is published by L. Fixter and C. Williamson titled “State of the art Review—Structural Health Monitoring” includes additional sensors used for monitoring bridge and civil structures as well as structures like aircraft. Yet another up-to-date article published by Papazian et al. in issue 29 of Journal of Fatigue, dated 2007, titled “Sensors for monitoring early stage fatigue cracking” explains use of eddy current, ultrasonic sensor and electrochemical fatigue sensor (EFS) in detail. These sensors also detect cracks that develop during life time of the structure.
There are also many commercially available fatigue sensors in the market. B. M. Phares from Material Technologies, Inc. reviews one of the innovative sensors called electrochemical fatigue sensor in a technology review article titled “The electrochemical fatigue sensor: A novel sensor for active fatigue crack detection and characterization” which is available from companies' website.
There have been numerous patent applications for sensors that detect health state of structures. U.S. Pat. No. 7,621,193 B2 by Fay et al. is a patent titled “Fracture detecting structural health sensor” which works by attaching a frangible membrane including a thin conductor sense loop is connected to a structural element to be tested. Failure of the structural element destroys the thin conductor sense loop and reveals the failure of the structural member. This invention is also designed to detect the crack after the crack occurs. The crack developed on the surface of the structure destroys the frangible loop and the sensor senses the existence of the crack this way.
Another patent application US 2009/031 by Goldfine et al. titled “Primary windings having multiple parallel extended portions” uses series of eddy current sensors to detect not only the existence but also the size of the crack developing under the sensor attached surface. Like the previous patent, this invention is also geared toward detecting cracks after the crack develops. As the crack progressively gets longer, more and more eddy current sensors sense the situation and inform the status of the crack
Majority of the sensors mentioned above, regardless whether they are chemical, optical, or electrical, they are all designed toward detecting the existence of the cracks. In the present invention, the approach is different. Rather than detecting the existence of the crack, the present invention intends to estimate the remaining service life of the structure and raise an alarm when the service life of the structural member gets closer to its limit. The sensor is wireless enabled which makes it possible to check the status of the sensor easily through as sensor network.
Reading sensors wirelessly is a well-researched subject and its techniques are well known in the state of the art. Lei et al. published an article in Chinese Journal of Aeronautics in 2009 in issue 22 titled, “Design an Experiment of PCT Network-Based Structural Health Monitoring Scanning System” explaining how such a wireless network can be constructed. Another article published by Mascerenas et al. titled, “Experimental Studies of Using Wireless Energy Transmission for Powering Embedded Sensor Nodes” in 2009 issue of the Journal of Sound and Vibration to describe how remote sensors with no batteries can be powered by remote RF energy in a bridge monitoring application.
Structural health monitoring applications requires a robust, reliable and easy to read sensors to monitor the fatigue state of structural members. What the expectations should be from an ideal structural health monitoring systems is researched by researchers from industry and academia. J. D. Acenbach summarizes these expectations in a review article titled “Structural health monitoring—What is the prescription?” in Journal of Mechanics Research Communications, 36 (2009) 137-142 as follows;
An ideal Structural Health Monitoring system should have such features:                Should have permanently installed microsensors,        On demand or continuous condition monitoring in real time should be possible,        Wireless transmission to central station,        Instantaneous interpretation of sensor data,        Detection of unacceptable material damage at critical high-stress locations,        Monitoring of growth of material damage into critical size,        Growth prediction by a probabilistic procedure,        Adjustments to growth prediction for actual damage state at prescribed intervals,        Probabilistic forecast of damage state for near term and of lifetime.        
Incorporation of all these attributes into a single structural health monitoring is not likely, but these are the desired features of from a state of the art, ideal structural health monitoring system.
Additionally, there are many technical challenges for ideal sensors to overcome. Ideally, the sensors used in structural health monitoring systems should be:                Small (microsensors), ligthweight.        Autonomously powered without any power wires,        Cheap, robust, maintainable, repairable and easy to install,        Accurate, known probability of detecting failure (POD).        Easy to properly couple to structure without causing any damage,        Suitable for wireless transmission to central station,        Can be densely distributed,        Capable of measuring both local and system-level response,        Designed to measure relevant damage parameters.        
The above requirements are summary of expectations from ideal structural health monitoring sensors. The present invention intends to address some of these requirements to come up with a lightweight, reliable sensor which enables monitoring health state of the structure whether it is an aircraft or a bridge.